Procedure for secondary recrystallization



3,090,711 PROCEDURE FOR SECONDARY RECRYSTALLIZATION Dale M. Kohler,Middletown, Ohio, assignor to Armco gteel Corporation, Middletown, Ohio,a corporation of nio No Drawing. Filed July 6, 1959, Ser. No. 324,915Claims. (Cl. 148-111) This invention relates to the manufacture ofsiliconiron sheet stock having a high degree of preferred orientationprimarily produced by secondary recrystallization from a suitablestarting material. The invention will be described in connection withthe manufacture of siliconiron sheet stock characterized preponderantlyby a (100) [001] crystal orientation, hereinafter referred to as a cubictexture, but is not necessarily so limited, as will hereinafter bepointed out.

It has been understood in the art that a silicon-iron sheet stock havinga high degree of cubic texture would be a useful material in theelectrical arts. It would be a material in which the cube faces of thecrystals lie parallel to or within a few degrees of parallelism to thesurface planes of the sheet stock, while the cube edges are generallyaligned with the rolling direction. As a consequence, such a materialcould be expected to have, and does in fact have, as high a permeability(or higher) in the straight grain or rolling direction as characterizessilicon-iron having the so-called cube-on-edge orientation, but also ahigh permeability in the transverse direction. Hence it would beespecially useful in the manufacture of transformers using corestampings, and in rotating electrical machinery.

While a number of ways of making a silicon-iron sheet stock having a.cubic texture have hitherto been suggested in the art, the onlyprocesses which have commerical economy are those in which a materialhaving, as a result of previous treatments and a final primaryrecrystallization, a reasonable number of grains oriented in the (100)[001] direction or near it, is subjected to a high temperature secondaryrecrystallization during which these grains grow at the expense ofgrains having a substantially difierent orientation, until the sheetattains a condition in which the cubic texture predominates.

Various ways have been suggested for producing a material of desiredgauge which is characterized by a reasonable number of cubic nuclei. Ina copendin-g application of the inventor and Martin F. Littmann,entitled Oriented Silicon-Iron and Process of Making It, Serial No.816,889, filed May 29, 1959, there is described a procedure in which thestarting material is commercially oriented silicon-iron having at (110)[001] or cube-onedge crystal orientation by Millers indices. Thisstarting material, by a series of cold rolling treatments andintermediate and final primary recrystallizations is carried through aseries of well defined derivative orientations to a condition in whichit has a relatively large number of crystal nuclei in the cubicorientation, whereupon it is subjected to secondary recrystallization.In another copending application entitled The Manufacture of SiliconlronHaving Cubic Texture, Serial No. 819,589, filed June 11, 1959, the sameinventors have described a process by which a material having asatisfactory number of cubic crystal nuclei may be produced from hotrolled strip stock in a series of two cold rollings and intermediate andfinal primary recrystallization treatments. The starting material forthis invention may be made by following steps of either of theseprocesses, or by any other processes which will yield a final gaugeproduct having a reasonable number of cubic crystal nuclei; and thereference to the said copending application is not given for purposes oflimitation but rather for the purpose of in- 3,090,711 Patented May 21,1953 corporating herein teachings which will result in the production ofthe desired starting material. The processes of the said copendingapplications are preferred because they work very well on air-meltedsilicon-iron stock (as produced in an open hearth furnace or arcfurnace, by way of example), although they may be used in connectionwith more expensive vacuum-melted stock if desired.

The [001] orientation which has been mentioned above implies not only analignment of the cube faces in parallelism or substantial parallelismwith the surfaces of the sheet stock, but also a general alignment ofthe cube edges in the direction of rolling. Other stocks can be made inwhich the cube faces are parallel or substantially parallel to the sheetstock surfaces, but in which the cube edges are not so aligned, butinstead are variously directed and may even be random in theirrelationship to the rolling direction. A stock of this general characteris described in the copending application of John M. Jackson, Serial No.706,091, filed December 30, 1957, and entitled Non-Directional OrientedSilicon-Iron. Such stocks may be made in any suitable way as by theprocess of the said copending application, by processes involving thestarting of columnar grains at the surfaces as when a phase change takesplace in the sheet stock at a substantially constant temperature and thelike. A product of this character, in which the cube edges have a randomorientation will not be characterized by permeabilities which are ashigh in the straight grain and cross grain directions as a materialhaving the cubic texture which has been described. But, the product willhave a substantially equal permeability in all directions includingthose intermediate the straight grain and cross grain directions. Theteachings of this application are applicable to the manufacture bysecondary recrystalhaving a silicon content of substantially 2.5 to4.0%,

and of a high degree of purity including low contents of carbon,sulphur, nitrogen, oxide inclusions and the like. Preferred silicon-ironmay contain from 2.90 to 3.30 silicon, a carbon content of the melt ofnot more than about .030%, which will later be reduced to less thanabout 005%, about .03 to .15 manganese, the remainder beingsubstantially all iron with a total oxide content which should not bemore than about 015% at the start of the routing, and which willpreferably be reduced to about .00 5 by the end of the routing.

Secondary recrystallization is a tricky procedure involving much morethan a high temperature heat treatment. In a copending application ofthe present inventor and John M. Jackson, entitled The Production ofOriented Silicon-Iron Sheets by Secondary Recrystallization, Serial No.813,289, filed May 14, 1959, there is described an annealing treatmentin an inert gas such as argon or helium, or in hydrogen, wherein verysmall quantities of polar compounds such as oxides of carbon or sulphur,or hydrogen sulfide, are entrained in the annealing gases during theperiod of the secondary recrystallization. It is believed that thesepolar compounds are absorbed or adsorbed on the surfaces of the crystalsin the sheet stock so as to satisfy the unsatisfied positive bondsthereat, the net result of the procedure being a shifting of the energylevels of crystals having various orientations in the sheet stock insuch a way that grains having the (100) plane parallel to the sheetsurface become the lowest in energy level, so that during the hightemperature heat treatment grains so oriented can grow vigorously asrespects other grains having different orientations. The use of thisannealing procedure is preferred for purposes of this invention; but theinvention is not necessarily confined thereto. It is applicable to anyhigh temperature annealing treatment capable of causing grains havingthe cubic texture to grow at the expense of other grains.

Various other factors, however, influence the occurrence and vigor ofthe secondary recrystallization phenomenon. It has been understood thatthe silicon-iron sheet stock itself should have high purity as above setforth and also that its surfaces should be clean, i.e. substantiallyfree from foreign material other than suitable annealing separators, andin particular free from oxide inclusions of substantial or massivecharacter (although a very thin film of iron oxide on the surfaces ofthe sheet stock may be tolerated). The surfaces of the sheet stockshould be as free as possible from any oxide materials which are notreducible in a high temperature heat treatment in hydrogen.

It is a primary object of this invention to provide a procedure and acondition of the material just before it is subjected to secondaryrecrystallization, both of which greatly facilitate the preferentialgrowth of low-energy oriented nuclei during secondary recrystallization,and in many instances make possible a perfection of the orientationproduced by the secondary recrystallization such as could not otherwisebe attained.

This and other objects of the invention which will be set forthhereinafter or will be apparent to one skilled in the art upon readingthese specifications, are accomplished by that procedure of which anexemplary embodiment will now be described.

In view of the requirements for cleanliness at the surfaces of the stockvarious treatments have been attempted. These include pickling,electropolishing, and others. The present inventor has hithertosuggested a treatment of the strip in a water solution containing aboutequal parts of orthophosphoric acid and hydrogen peroxide. Someimprovement in the action during secondary recrystallization was noted;:but it was not known whether this improvement was due to a cleaningaction or to a chemical addition of some kind absorbed or adsorbed uponthe surfaces of the sheet stock.

-It has now been found that, aside from the nature and purity of thebase metal as hereinabove defined, freedom of its surfaces from foreignmatter and inclusions especially of materials which are not reducible inhydrogen at high temperatures, and the use of polar compounds in theannealing atmosphere as taught in the copending application lastmentioned above, an extremely important single factor in an otherwisesuitable heat treatment for secondary recrystallization, is a physicalas distinguished from a chemical condition of the surfaces of the sheetstock. This physical condition has been discovered to be one ofsmoothness as hereinafter defined. It has been found that a physicallysmooth surface on a cold rolled sheet stock is a very valuable aid tosecondary grain growth. The smoothness referred to does not necessarilyrequire that the sheet surfaces be uniplanar. A rather high peak tovalley variation as determined by a profilometer measurement can betolerated if the transition from peaks to valleys is gradual andoccupies a substantial dimension in the general plane of the sheetsurface. On the other hand variations in the sheet surfaces which aresharp or distinctly angular have been shown to be detrimental. A scratchdeliberately formed on a surface of the sheet can be shown to impairdrastically the grain growth action upon secondary recrystallization.

While it is conceivable that a satisfactory surface condition could bebrought about by electropolishing or chemical polishing, this wouldrequire a relatively long continued treatment, and would involve suchgreat expense as to impair the commercial value of the product.

It has been found that the best and least expensive way of securing theoptimum surface condition is to cold roll the product with smoothpolished rolls. Under these conditions the surfaces of the sheets becomesmooth as herein defined and acquire a high luster. Under theseparticular conditions, the peak to valley measurement as determined by aprofilometer also becomes an index of the attainment of the desiredsmoothness, since the manner in which rolls are polished tends topreclude the existence of widely disparate peaks and valleys.

In a series of tests, it was found that the rolling of the material onpolished rolls as hereinafter defined produced a far greater improvementin the desired secondary grain growth (as compared with materials rolledon mills having rolls with normal grinding) than did a chemicalpolishing of the sheets as ordinarily rolled. In other words samples ofsilicon-iron cold rolled on polished rolls showed much more cubicsecondary grain growth in the same final annealing operation than didsamples rolled on conventional mill rolls, or conventionally rolledsamples after chemical polishing.

The following table shows the results on secondary grain growth of theuse of rolls which have had difierent finishing treatments:

urements .2-1 Bufiing.

1-5 Grinding with cork or shellac bonded abrasive wheel.

5-12 Grinding with commonly used lfiO grit grinding wheel. 7 12-100Grinding plus light pangborn of roll.

The profilometer measurements given are those characteristic of thefinishing treatments. The effect on grain growth in the secondaryrecrystallization is necessarily given in qualitative terms due tovariations in specific samples, especially where the samples arerelatively small; but this classification is the result of extendedexperience; and it is believed that the effect of the rolls on thesiliconiron sheet stock is the factor producing the greatestirnprovement in the attainment of a high degree of cubic secondarygrowth, assuming that the secondary recrystallization is otherwisecarried on under favorable conditions.

Hitherto the ordinary commercial roll finishes for the rolling ofsilicon-iron in orientation processes have had profilometer readingsvarying fiom about 10 to about microinches.

In the practice of the invention, while all of the cold rolling passesgiven the material may be carried on with polished rolls, it is notnecessary to do this. The cold rolling reduction in stages prior to thefinal stage may be carried on with rolls having commercial finishes. Thefinal stage of cold rolling, if desired, may employ rolls withcommercial finishes for the greater part of the reduction; but thepurposes of this invention will be attained if the last part of thefinal cold rolling is carried on with polished rolls. An excellentefiect can be attained in a single final pass through polished rollsproviding the reduction in that pass is great enough to produce thedesired smooth surface i.e. a reduction of 2% or more.

In general excellent silicon-iron sheet stock having preponderantly thecubic crystal orientation can be produced from a material which in acondition of final primary recrystallization is characterized by analignment of at least about 70% of the cube edges to within 20 of therolling direction and an angular relationship of the cube faces of asubstantial number of the grains to within at most about 5 ofparallelism with the sheet surfaces. The skilled worker in the art willunderstand that primary recrystallization will occur at temperaturesroughly between l400 and 1700 F., and that secondary recrystallizationrequires a temperature of substantially 1900 to 2300 F. Since theprimary recrystallization occurs "cry rapidly, it is usual for economicreasons to combine a final primary recrystallization with the secondaryrecrystallization, the primary recrystallization occurring during theheating up of the stock to the high temperatures required for thesecondary recrystallization.

Modifications may be made in the invention without departing from thespirit of it. The invention having been described in certain exemplaryembodiments, what is claimed as new and desired to be secured by LetterPatent is:

1. In a process of producing silicon-iron sheet stock characterizedpredominantly by an orientation of the crystals in which the cube facesthereof are substantially parallel to the surfaces of the sheet stock,and including the steps of producing a silicon-iron sheet stock which,after a primary recrystallization will be characterized by a substantialnumber of grains having their cubic faces so oriented, the production ofsaid stock involving a final cold rolling treatment immediately precedinsaid primary recrystallization, the improvement which consists in thatthe said cold rolling treatment is characterized in the final part atleast by the reduction of the silicon-*on sheet stock between rollshaving a profilometer reading not greater than about 5 microinchesbefore subjecting the stock so produced to a secondary recrystallizationtreatment at high temperatures.

2. In a process of producing silicon-iron sheet stock characterizedpredominantly by a cubic orientation of the crystals, and including thesteps of producing a silicon-iron sheet stock which, after a primaryrecrystallization will be characterized by a substantial number ofgrains having cubic orientation, the production of said stock involvinga final cold rolling treatment immediately preceding said primaryrecrystallization, the improvement which consists in that the said coldrolling treatment is characterized in the final part at least by thereduction of the silicon-iron sheet stock between rolls having aprofilometer reading not greater than about 5 microinches beforesubjecting the stock so produced to a secondary recrystallizationtreatment at a high temperature.

3. The process claimed in claim 2 wherein the said secondaryrecrystallization is carried on at a temperature substantially between1900 and 2300 -F. in an atmosphere of non-oxidizing gas containing asmall quantity of a polar compound selected from a class consisting ofoxides of carbon and sulfur and hydrogen sulfide.

4. A process of producing silicon-iron sheet stock having a high degreeof cubic texture which comprises subjecting a silicon-iron materialcontaining substantially 2.5 to 4.0% silicon, a carbon content notsubstantially greater than .005% and an oxide content not substantiallygreater than .005 to a cold rolling and primary recrystallizationtreatment in which the crystals thereof are caused to assume anorientation in which at least about of the cube edges are aligned within20 of the rolling direction, and in which a substantial number of thegrains have their cube faces tilted to within substantially 5 ofparallelism With the sheet stock surfaces, and thereafter subjecting thestock to a secondary recrystallization, the improvement which consistsin accomplishing at least the latter part of said cold rolling by meansof polished rolls having a profilimeter reading of about .2 to 1microinch.

5. The procedure claimed in claim 4 wherein said secondaryrecrystallization is carried on at a temperature or" substantially 1900to 2300" in a non-oxidizing atmosphere containing a small amount of apolar compound selected from the group consisting of oxides of carbonand sulfur and hydrogen sulfide.

References Cited in the file of this patent UNITED STATES PATENTS2,455,632 Williams Dec. 7, 1948 2,867,558 May Jan. 6, 1959 FOREIGNPATENTS 1,009,214 Germany May 29, 1957 OTHER REFERENCES Metals Handbook,American Society vfor Metals, Cleveland, 1948 edition, page 56.

1. IN A PROCESS OF PRODUCING SILICON-IRON SHEET STOCK CHARACTERIZEDPREDOMINANTLY BY AN ORIENTATION OF THE CRYSTALS IN WHICH THE CUBE FACESTHEREOF ARE SUBSTANTIALLY PARALLEL TO THE SURFACES OF THE SHEET STOCK,AND INCLUDING THE STEPS OF PRODUCING A SILICON-IRON SHEET STOCK WHICH,AFTER A PRIMARY RECRYSTALLIZATION WILL BE CHARACTERIZED BY A SUBSTANTIALNUMBER OF GRAINS HAVING THEIR CUBIC FACES SO ORIENTED, THE PRODUCTION OFSAID STOCK INVOLVING A FINAL COLD ROLLING TREATMENT IMMEDIATELYPRECEDING SAID PRIMARY RECRYSTALLIZATION, THE IMPROVEMENT WHICH CONSISTSIN THAT THE SAID COLD ROLLING TREATMENT IS CHARACTERIZED IN THE FINALPART AT LEAST BY THE REDUCTION OF THE SILICON-IRON SHEET STOCK BETWEENROLLS HAVING A PROFILOMETER READING NOT GREATER THAN ABOUT 5 MICROINCHESBEFORE SUBJECTING THE STOCK SO PRODUCED TO A SECONDARY RECRYSTALLIZATIONTREATMENT AT HIGH TEMPERATURES.